A guide rod, which has a diameter expandable tip end portion and restricts the oscillation of a connecting rod and a piston, is inserted into a cylinder bore via an opening adjacent to a crank chamber of the cylinder bore, and the piston is inserted into the cylinder bore via a cylinder head mounting side opening with the connecting rod positioned at a leading end. The diameter of the tip end portion of the guide rod is increased in a hole of a semi-circular arc-shaped end portion positioned at tip end of the connecting rod in the insertion direction, and thus the connecting rod is engaged with the guide rod. The guide rod moves and the connecting rod is pulled into the cylinder bore until the semi-circular arc-shaped end portion of the connecting rod sits on a crankpin of a crankshaft.